The present invention concerns a method of assembling sections of support legs of an offshore oil production platform.
Offshore oil production platforms generally have legs, for example three legs, resting on the seabed and a hull movable along and adjustable in height along the legs and carrying production equipment and crew quarters, among other things.
The platform is constructed and then floated out to the drilling or production site, the legs are lowered into contact with a support structure or the seabed, and then, supported on the legs, the hull is raised above sea level to a height such that it is out of range of the highest waves.
The hull is therefore movable along the support legs of the hull by means of drive mechanisms accommodated in a support framework supported by the hull and well known to the skilled person as the "jack-house".
The drive mechanisms of each support leg each include at least two opposed motor-gearbox units driving output pinions that cooperate with opposed racks mounted on the legs.
To this end, each leg passes through the hull and the corresponding support framework and is formed of superposed sections welded together.
Each section can have a height in the order of 18 meters and is generally made up of three or four vertical booms linked together by a truss of metal beams or by box-sections.
Each boom is formed by a rectangular plate and by stiffeners in the form of half-shells each welded to one of the main faces of the plate.
The rectangular plates have teeth on their lateral face which form the diametrally opposed racks adapted to cooperate with the output pinions of the drive mechanisms.
These platforms are constructed in a shipyard by first assembling the various component parts of the hull and mounting the drive mechanism support framework on the hull where each leg passes through it.
The assembled hull rests on the ground of the shipyard on appropriate support booms.
Until now two methods have been used to assemble the sections of each leg and mount them on the hull.
The first method uses a tall crane to lift a first section of each leg to a position vertically above the corresponding passage in the hull and the support framework and then to lower it progressively in order to. engage it vertically in this passage.
A second section is then lifted and positioned over the first section and these sections are then welded together.
However, this method has drawbacks because of the height of the sections, which is approximately 18 meters for legs with a height between 70 and 170 meters.
It requires the use of at least one tall crane to lift the sections one above the other and the sections are welded together at a great height, particularly in the case of the final sections, which makes the various operations difficult, complicated and subject to the vagaries of inclement weather.
Thus the operations of mounting and assembling the legs must be interrupted in the event of high winds because of the danger to personnel.
The second method consists in assembling the various components of the hull at a sufficient height above the ground of the shipyard to provide a space enabling the vertical sections to be moved under the hull.
By virtue of this space, the first section of each leg is brought vertically below the hull and then raised to position it in the passage formed in the hull and the drive mechanism support framework.
The second section is then placed under the first section and these two sections are welded together.
These various operations are repeated to construct each leg.
This assembly method also has drawbacks, however.
It requires the construction of large supports, concrete supports, for example, to support the hull, and when the various components of the hull are assembled they must be lifted, which increases the number of handling operations and consequently the cost of building the hull.
Moreover, personnel are required to ascend the distance between the hull and the ground several times a day.